The prior art teaches equipping vehicles with “variable displacement,” “displacement on demand,” or “multiple displacement” internal combustion engines in which one or more cylinders may be selectively “deactivated,” for example, to improve vehicle fuel economy when operating under relatively low-load conditions. Typically, the cylinders are deactivated through use of deactivatable valve train components, such as the deactivating valve lifters as disclosed in U.S. patent publication no. US 2004/0244751 A1, whereby the intake and exhaust valves of each deactivated cylinder remain in their closed positions notwithstanding continued rotation of their driving cams. Combustion gases are thus trapped within each deactivated cylinder, whereupon the deactivated cylinders operate as “air springs” to reduce engine pumping losses.
When vehicle operating conditions are thereafter deemed to require an engine output torque greater than that achievable without the contribution of the deactivated cylinders, as through a heightened torque request from the vehicle operator based upon a detected intake manifold air pressure representing a current engine load, the deactivatable valve train components are returned to their nominal activated state to thereby “reactivate” the deactivated cylinders. More specifically, under one prior art approach, a torque request or torque demand signal, as determined, for example, from current accelerator pedal position and current engine speed, is compared to a mapped value for available engine torque at that engine speed. A value for a torque “reserve” representing an output torque “cushion” during a subsequent transition to a full-cylinder-activation mode with no more than a negligible torque disturbance (generally imperceptible to the vehicle operator) is also calculated or provided. When the torque request exceeds the mapped threshold value less the reserve threshold, the engine control module initiates a “slow” transition out of the cylinder-deactivation engine operating mode. These “slow” transitions, intended to feature only those transition torque disturbances that are generally imperceptible to the vehicle operator, are to be distinguished from “fast” transitions that are typically triggered in response, for example, a torque request that well exceeds the available engine torque, under which conditions a noticeable torque disturbance is perhaps even desirable as feedback to the vehicle operator.
Unfortunately, because the prior art “trigger” for such “slow” transitions back to a full-displacement engine operating mode is based upon detected manifold air pressure, it will be appreciated that the prior art approach may specify continued engine operation in a partial-displacement mode that might otherwise generate unacceptable levels of vehicle noise, vibration, and harshness (NVH) determinations. Further, such prior art approaches necessarily require corrections to the detected manifold air pressure, for example, for ambient barometric pressure and temperature, thereby increasing the complexity of the calculations from which a maximum engine output torque in partial-displacement mode is derived, while further requiring such additional engine hardware as a barometric pressure sensor.